Incidence of the Ascarid Porrocaecum decipiens in the Stomachs of Three Species of Seals along the Southern Canadian Atlantic Mainland

Abstract

Identifications were made of ascarids collected from 1948 to 1956 from the stomachs of 318 harbour seals (Phoca vitulina), 812 harp seals (P. groenlandica), and 127 grey seals (Halichoerus grypus) killed in several localities along the Atlantic coast of Canada. Porrocaecum and 2 other genera of ascarids, tentatively identified as Anisakis and Contracaecum, were present in each species of seal. All 112 male Porrocaecum that were identified to species were P. decipiens.Almost all stomachs of harbour and grey seals at all seasons and in all localities contained P. decipiens. Considering all localities, the overall average incidence was about 20 adult P. decipiens in harbour seals and about 100 in grey seals.P. decipiens was normally much rarer in the stomachs of harp seals than in the other seals. It appeared most frequently in harp seals taken around the Magdalen Islands in April and May. The incidence of adult P. decipiens was much lower during the breeding season of the seals and during their southward and northward migrations within the Gulf of St. Lawrence. The incidence of P. decipiens in harp seals from the east coast of Newfoundland was very low.The relative importance of each species of seal as a vector of P. decipiens is as follows. The harbour seal is the most important in the Bay of Fundy and along the southwestern coast of Nova Scotia. Elsewhere harbour and grey seals occur in approximately equal numbers. In such localities the grey seal is probably more important than the harbour seal. The harp seal is about 100 to 200 times as numerous, while in the southwestern Gulf of St. Lawrence, as the combined populations of harbour and grey seals there. Despite this great numerical superiority the harp seal does not appear to be more important than the other seals as a host of P. decipiens.The relation between each species of seal and the incidence of larval P. decipiens in Atlantic cod (Gadus callarias) is discussed.